This invention relates to pressure fault relay apparatus for use with liquid-filled electrical equipment. In normal operating conditions the relay allows the electrical apparatus to function without disconnecting the electrical equipment from the electrical supply. Relays are currently available having pressure sensing mechanisms for determining pressure changes associated with internal fault conditions and for providing electrical output signals to operate disconnect controls.
When pressure relays are used with liquid filled transformers the relay must be able to differentiate between a true internal fault pressure wave and a pressure disturbance caused by through faults or faults occurring outside the electrical equipment. In order to insure that the relay operates immediately when internally created pressure disturbances occur, the pressure threshold for operating the relay is set low enough such that externally created through-fault disturbances can sometimes cause the relay to operate.
The false operation of the relay caused by through-fault pressures of a temporary nature can cause unnecessary power outages to occur that are very expensive to investigate and reconnect the equipment to the line.
The purpose of this invention is to provide a simple mechanical means for discriminating between an internal generated fault and an external generated through fault so that electrical signal indication is made for an internal fault pressure disturbance only.
U.S. Pat. No. 3,898,404 describes a sudden pressure relay having a plurality of flow restricting baffles. The relay assembly is mounted in the gas space above the liquid medium and contains a plurality of disc-shaped baffles for discriminating between sudden and between longer term thermal pressure increases.
U.S. Pat. No. 3,090,848 discloses a fluid-pressure actuated switch containing a perforated metal washer for stabilizing the relay against operating on momentary pressure fluctuations.
When perforated discs, similar to the devices disclosed in the aforementioned U.S. patent, are employed in a pressure relay for protecting liquid filled electrical equipment, the pressure becomes restricted to such an extent that in some instances the time between the pressure disturbance and relay actuation is substantially delayed. In cases where an internal fault exists, such as the type occurring when the transformer winding forms an electrical arc, substantial damage to the transformer can occur before the relay becomes actuated. A further purpose of this invention is to provide means for a fault pressure relay to be relatively undisturbed by temporary through fault pressure yet become readily actuated upon the occurrence of an internal pressure fault.